1. Field of the Invention
This invention relates generally to the field of dry ice manufacturing, and more particularly to a method and apparatus for producing blocks of dry ice. 2. Description of Related Art
Dry ice is the solid state of carbon dioxide (CO2). There are a vast array of applications for dry ice, including the processing and preservation of meats and other foods. Dry ice is the preferred means of cooling in such applications, since it imparts no color, odor, or taste, and has no lingering deleterious effect on the food. Dry ice also is desirable for the processing of food because its sublimes directly from the solid state to the gaseous phase, leaving no residue behind after yielding its cooling effect; therefore, no clean-up or removal of residual liquid is required. Furthermore, CO2 is neither toxic, poisonous, reactive with other chemicals, nor flammable.
In its solid state, at standard temperature and pressure, carbon dioxide has a constant and stable temperature of xe2x88x92109.33xc2x0 F. Carbon dioxide is normally transported in its liquid state, and stored in refrigerated vessels at a pressure of about 305 psia, and a corresponding temperature of about 0xc2x0 F.
Once the liquid CO2 reaches the manufacturing facility, dry ice is generally formed into one of the two final forms, blocks of dry ice or smaller pellets. Large blocks of dry ice typically are shipped long distances or stored for extended periods, as pellet size pieces sublimate faster.
The basic process for making block dry ice from liquid carbon dioxide has long been known. Dry ice block manufacture has changed little in the last sixty-five years or so. Over this time, various kinds of apparatuses for carrying out this basic process have been devised. Typical of such conventional apparatus is that which is the subject of Great Britain Complete Specification No. 433,018, accepted Aug. 7 1935. Commonly, present ice block manufacturing incorporates the Southwark-Baldwin press. This machine can produce a 220-pound block of ice. This type of dry ice block press utilizes what is conventionally referred to as a liquid injection process.
The liquid injection process injects liquid CO2 and a binding agent at a pressure above the triple point of CO2 into the top of a compression chamber. Liquid CO2 is supplied to the compression chamber from a remotely located CO2 supply. When the injection process is complete, the liquid CO2 in the chamber drops below the triple point and undergoes a phase change, thus producing solid CO2 (snow).
The amount of liquid CO2 injected into the compression chamber does not produce a complete block of dry ice until the chamber reaches a minimum equilibrium temperature. For example, the minimum equilibrium temperature of TOMCO2 ice machines is approximately xe2x88x9250xc2x0 F. This temperature naturally varies between press types. This process of reaching a minimum equilibrium temperature is similar to the cool down period ice machines go through before they start producing blocks that are considered complete.
The liquid CO2 is permitted to flash through an expansion device and enter the compression chamber over the triple point pressure from a nominal storage pressure, for example, 100 psia, wherein part of the liquid will turn into gas and part of the liquid will solidify. The proportionate amounts of gaseous CO2 and solid CO2 depend on the pressure and temperature of the liquid CO2 fed into the chamber. The lower the pressure and temperature, the greater the proportion of solid CO2 formed as a result of the free expansion. Liquid CO2 initially at about 300 psia and approximately xe2x88x928xc2x0 F., when allowed to rapidly expand to atmospheric pressure, yields approximately 1.0 pound of dry ice as snow and about 1.5 pounds of vapor.
The gaseous CO2 is released through an exhaust port typically located near the top of the chamber, and returned to either a recovery unit or the atmosphere. A hydraulic press then compresses the CO2 snow until a preset hydraulic pressure is obtained.
A timer generally determines the amount of liquid CO2 injected into the chamber. However, with a timer, there is no compensation for the loss of CO2 due to the chamber temperature (i.e., the internal heat of steel). Further, this conventional dry ice process does not incorporate controls either to vary the ice block size, or to provide blocks with uniform block density.
Sometimes, blocks of dry ice from a block press are reduced to a smaller size that can more easily be handled and used in many types of applications. Other machines, for example the dry ice pelletizer, produces dry ice pellets. Dry ice pellets are easily packaged by the manufacturer and subdivided by the consumer into convenient portions for use.
Several disadvantages of conventional dry ice manufacturing processes are known, and include: the requirement of mixing a binding agent with the liquid CO2 prior to injection into the compression chamber; the incomplete and inefficient vapor removal from the compression chamber; the low vapor exhaust rates; the production of blocks having nonuniform densities; and the production limit of single-sized product. Therefore it can be seen that there is a need in the art for an improved dry ice block press that overcomes these and other prior art deficiencies.
Briefly described, in a preferred form, the present invention is a dry ice block manufacturing process including a CO2 storage vessel to store and deliver the liquid CO2, a dry ice production assembly to transform the stored CO2 into ice blocks, an automated analysis system.
The CO2 storage vessel incorporates a supply line to supply the dry ice production assembly with liquid CO2. A supply flow meter can be located in the flow path of the supply line.
The dry ice production assembly of the present invention comprises a compression chamber, a compressing mechanism and a heating element. Liquid CO2 flows from the CO2 storage vessel, through the supply line and flow meter, and then introduced into the compression chamber through one or more injection ports. The liquid CO2 injected into the chamber then changes into gaseous and solid forms of CO2. The compression chamber also has one or more venting ports for the release of built-up pressure, in the form of CO2 vapor, in the chamber, as the liquid CO2 proceeds through phase changes.
The compressing mechanism of the dry ice production assembly then compresses the resulting CO2 snow in the compression chamber into a single mass of solid dry ice. The compressing mechanism includes a piston and piston rod.
Heat is then applied by the heating element of the dry ice production assembly to the chamber walls in proximity to the dry ice block after compression in order to facilitate vapor and product removal from the chamber without dwell time. The introduction of heat also contributes to uniform block density and removes the need to combine the injected CO2 with a binding agent as is presently done in conventional block manufacturing.
The automated analysis system enables the dry ice production assembly to directly connect to the CO2 storage vessel, and controls the entire process.
The present dry ice block manufacturing process incorporates numerous novel improvements over conventional press methods. For example, a first advantage of the press of the present invention is a chamber retention assembly of the compression chamber that enables free expansion and contraction of the chamber with temperature changes within the chamber. This freedom of movement prevents damage to the chamber caused by the stresses and strains due to temperature changes.
The chamber can further include filter media placed over one or more of the venting ports in order to maximize the vapor exhaust rate of CO2 from the chamber. Filters over the venting ports allow such a rapid exhaust rate without traditional concerns including the loss of snow into the exhaust piping. Without the present filters, escaping snow would accumulate and damage machinery downstream of the chamber, such as a recovery.
Yet, another improvement is an ultra high molecular weight (UHMW) polyethylene surface plate on the piston of the compressing mechanism to prevent the top of the formed ice block from being damaged and sticking to the piston during compression.
A fourth improvement involves the heating element of the dry ice production assembly; the heating element capable of transferring heat to the ice block formed inside the compression chamber. The heat sublimates a portion of the block, enabling trapped CO2 vapor in the block to escape the block and exit the chamber. The heat also enables the present press to operate without dwell time. This addition of heat is one of several steps that permits the present press to consistently provide uniform blocks of ice through an entire run.
The automated analysis system of the present invention permits the production of uniform blocks by monitoring the temperature of the various components of the dry ice production assembly, including the compression chamber and the compressing mechanism. Based on changes in temperature of the chamber and of the related components, the amount of CO2 injected into the chamber is adjusted to compensate for these temperature changes. As temperatures rise, so too does the amount of injected liquid CO2 (by weight) to form a constant block (by weight) of ice.
One consequence of the constant automated monitoring and adjusting of the present process is the ability to program the present press with a predetermined desired block weight, and to have such uniform blocks manufactured time and again during the entire process run, without regard to human oversight and adjustment to the process.
Yet a seventh improvement over the prior art is the ability of the automated analysis system to monitor CO2 usage via the supply flow meter of the CO2 storage vessel that can tally the amount of CO2 that has passed through the flow meter.
Accordingly, it is an object of the present invention to provide an improved dry ice block press and method of ice block production.
It is a further object of the present invention to provide a block press incorporating a heating element to transfer heat energy to the ice inside the chamber in order to sublimate a micro-thin layer of the dry ice block, which limits or eliminates dwell time.
It is another object of the present invention to provide a press with filter media placed over the venting ports of the compression chamber.
It is another object of the present invention to provide a press that produces blocks of dry ice with uniform block densities over a range of block sizes.
These and other objects, features and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.